Refers collectively to all types of Digital Subscriber Lines, the two main categories being  ADSL and SDSL. xDSL is the term for the Broadband Access technologies based on Digital Subscriber Line (DSL) technology.  “x” signifies that there are various flavors of DSL. It provides always-on, high-speed data services over existing copper wires to residences & businesses, POTS service and DSL coexist on same copper line,  Lower rate xDSL (up to 1.5 Mbps) is gaining popularity in the residential market; will get faster and cheaper high performance xDSL (up to 52 Mbps) targets business and high-end users.  

ADSL

ADSL is the new modem technology to converge the existing twisted pair telephone lines into the high−speed communications access capability for various services. Most people consider ADSL as a transmission system instead of a modification to the existing transmission facilities. In reality, ADSL is a modem technology used to transmit speeds of between 1.5 Mbps and 6 Mbps under current technology. It is stated that in the future ADSL will support speeds of about 8.192 Mbps. This definition of the higher range of ADSL speeds is one that is yet to be proven; however, with changes in today's technology one can only imagine that the speeds will be achievable.

IDSL

DSL refers to a pair of modems that are installed on the local loop (also called the last mile) to

facilitate higher speeds for data transmission. Network providers do not provide a line; they use the existing lines in place and add the DSL modems to increase the throughput. DSL modems offer duplex operations — transmission in both directions at the same time. The speed of a DSL modem may be 160 Kbps on copper at distances up to 18K using the twisted pair wires. The bandwidth used is from 0 to 80 kHz, as opposed to the arbitrarily limited 0 to 3300 Hz on a voice line. This is the IDSL using the 144 Kbps full duplex, which gives us what is known as the Basic Rate Interface (BRI).

SDSL

The goal of the DSL family was to continue to support and use the local copper cable plant.

Therefore, the need to provide high−speed communications on a single cable pair emerged. Most local loops already employ single cable pair today; thus, it is only natural to assume the providers would want this capability. SDSL was developed to provide high−speed communications on that single cable pair but at distances no greater than 10K. Despite this distance limitation, SDSL was designed to deliver 1.544 Mbps on the single cable pair. Typically, however, the providers offer SDSL at 768 Kbps. This creates a dilemma for the carriers because HDSL can do the same things as SDSL.

xDSL Coding Techniques

Many approaches were developed as a means of encoding the data onto the xDSL circuits. The

most common are Carrierless Amplitude Phase Modulation (CAP) and discreet multitone (DMT)

modulation. Quadrature with Phase Modulation (QAM) has also been used, but the important part is the standardization. The industry, as a rule, selected DMT, but several developers and providers have used CAP. It is, therefore, appropriate to summarize both of these techniques. The SHDSL technology uses a trellis−coded pulse amplitude modulation (TCPAM) technique to gain the benefits of the single−pair services or two−pair service.

Provisioning xDSL

In the following figures, the various architectures of the xDSL implementations are shown. The point to remember here is the goal of xDSL is to use the existing copper infrastructure and improve the speed and throughput on the installed base of wires. Consequently, the installation process attempts to minimize the added equipment (particularly at the customer's premises) and the labor required to get the equipment installed.

references:

http://www.xilinx.com/esp/wired/optical/collateral/DSL.pdf

http://www.webopedia.com/TERM/X/xDSL.html

Broadband Telecommunications Handbook (Regis J. "BUD" Bates)